Organic Solar Cells Green River Project Silicon Cells Silicon - PowerPoint PPT Presentation

Organic Solar Cells Green River Project

Silicon Cells • Silicon semiconductors Advantages: •Efficiencies •Lifetimes Disadvantages: •High manufacturing costs •Inflexible http://en.wikipedia.org

Organic semiconductors Advantages: •Light, thin, and flexible •Potentially cheap manufacturing Current Disadvantages: •Efficiencies •Lifetimes Georgia Institute of T echnology

Organic Photovoltaic Devices Konarka Plextronics Home Power Generation San Francisco bus shelters

How does the Organic Photovoltaic Device work? Photons incident on the clear glass results in the formation of excitons. The excitons separate and the electrons are transported to the cathode and holes to the anode giving rise to a voltage and therefore a current when the circuit is completed. Hole transporter – P3HT PEDOT-PSS helps to lower the Electron transporter – PCBM (soluble energy barrier between the ITO and the derivative of PCBM) P3HT-PCBM layer. http://depts.washington.edu/cmditr/mediawiki/index.php?title=Organic_Solar_Cells

Two approaches to OPVs • Form a donor-acceptor bilayer using vacuum deposition. • Bulk Heterojunction (BHJ) – Maximize interface between donor and acceptor. – Polymer-based, solution processable – Low cost, light-weight, flexible – Process active layer in a single step (much easier than vacuum deposition!) • Inkjet printing/Spin Coating/Roller casting

Bulk Heterojunction (BHJ) Janssen, R.A.J.; Hummelen, J.C.; Sariciftci, N.S. MRS Bulletin. 2004 , 30, 33 .

Basic requirements for a good OPV • Device must be good at absorbing light (absorption coefficient) • Device must generate the greatest number of charge carriers with minimum concomitant loss of energy! • Device must be capable of transporting these charge carriers (holes and electrons) to the respective electrodes at a maximum rate. (charge carrier mobility) • Morphology (how do PCBM and P3HT interface with each other?)

Energy conversion in Excitonic Solar Cells • General Mechanism Polymer-Fullerene Composite Solar Cells, Thompson and Frechet, Angew. Chem. Int. Ed. 2008, 47, 58-77

Factors that affect performance • Choice of components in the active layer – Determines the electronic interactions that cause the formation of the excitons, diffusion, dissociation and charge transport. • Morphology (the AFM tells us this) – Determines the physical interactions between the components of the active layer i.e. can the exciton diffuse far enough before it recombines? How well can the excitons contribute to the current?

Choice of materials P3HT-PCBM in 25mg:15mg ratio dissolved in 1mL of dichlorobenzene • Electron Acceptor Fullerene [6, 6] phenyl-C61-butyric acid methyl ester (PCBM) O OMe • High electron affinity • Transports charge easily. S Br H • Electron Donor S n/2 P3HT PCBM P3HT Electron Acceptor Electron Donor [Poly (3 – hexylthiophene)] Typical efficiencies of 5% (this is the HIGH end)

Optimum donor material for PCBM • Need to balance donor LUMO – acceptor LUMO levels • Maximize donor HOMO and acceptor LUMO difference for high V OC Ideal donor: LUMO=3.9 eV & HOMO =5.4 eV Model predicts 20% PCE Dennler & Brabec, Adv. Mater. 2009 , 21, 1323

Characterization of OPVs FF x V oc x J sc Electrical output power Solar power conversion efficiency  (%) = = Sun light power Sun light power Maximum { J (mA/cm 2 ) x V (v)} = 100 (mW/cm 2 ) ( = 1 kW/m 2 , A.M 1.5) by Solar Simulator Current density J (mA/cm 2 ) = Current (mA) / Active area (cm 2 ) V oc V ( v ) Jsc Maximum { J x V } V oc Maximum { J x V } Fill Factor (FF) = V oc x J sc Jsc J (mA/cm 2 )

Anode: Clean and cut ITO coated glass and anneal at 140 0 C for 10 minutes. Then Spin coat with PEDOT/PSS (2000rpm for 30 seconds). Anneal at 140 0 C for 15 minutes. Spin coat with the active layer (P3HT-PCBM) and annealed all but four samples again at 140 0 C for 10 minutes. Cathode: Apply a small quantity of GaIn on one corner and encircle with thin ring of epoxy. ITO glass with Finally, invert the cathode and gently place on the anode. Polymer Layer Need offset to attach alligator clips! Previously unannealed samples are now annealed. Epoxy ring GaIn Eutectic GaIn Eutectic ITO coated glass Optical Adhesive (we used 5 minute epoxy) ITO coated glass

Finished device Substrate [Glass] Epoxy (sealant) ITO Same anode but use a liquid Metal alloy (GaIn eutectic) Photon Absorbing Layer Work function ~ 4.2eV [PCBM-P3HT] as the cathode. Gallium Indium Eutectic (Cathode) (PEDOT-PSS Conducting Layer ) Note: same work function as the Al electrode but NO Anode (ITO) VACUUM DEPOSITION!! Substrate (glass) http://www.ece.ncsu.edu/oleg/files-wiki/c/cc/ModuleThreeProcedures.pdf

How do we increase efficiency? • Fine tune the electronic interactions between the polymeric donor (P3HT) and the fullerene acceptor (PCBM). • Device Architecture • Morphology • Processing

Role of the Active Layer • Governs the mechanism for light absorption • The exciton diffusion • The charge transport • Charge collection occurs in the interface between the eutectic and the active layer. • Performance depends on Choice of Active layer components Morphology of the sample (controls the physical interaction between the donor and the accepteor)

Electronic interactions & Morphology • Other electronic schemes – engineering the LUMO-LUMO difference and the HOMO(donor) and LUMO (acceptor) levels etc. • Mean domain size should be approximately the diffusion length (5 – 10nm) • Phase segregation for effective charge pathways.

Morphology • Intrinsic – Factors that are inherent to the fullerene and the polymer and how they interact. • Extrinsic – Solvent choice (toluene vs. dichlorobenzene) – Ratio of P3HT and PCBM – Thickness of active layer (spin speeds) – Spin-coating/roller casting/ink-jet printing – Solvent evaporation rate – annealing times

Anode: Clean and cut ITO coated glass. Spin coat with PEDOT/PSS (4000rpm for 30 seconds). Anneal at 140 0 C for 15 minutes. Spin coat with the active layer (P3HT-PCBM - 4000rpm for 60 seconds) and annealed again at 140 0 C for 10 minutes. Cathode: Apply a small quantity of GaIn on one corner of cleaned ITO glass substrate and encircle with thin ring of epoxy. ITO glass with Finally, invert the cathode and gently place on the anode. PEDOT-PSS and Active Need offset to attach alligator clips! Layer Epoxy ring GaIn Eutectic GaIn Eutectic ITO coated glass Optical Adhesive (we used 5 minute epoxy) ITO coated glass

Goal of GRCC projects • Pick one parameter and optimize the organic solar cell for maximum efficiency (as measured using J-V data). • AFM data to study the morphology of these samples and correlate it to performance would be a “nice thing to do” (currently a challenge), • Optical characteristics of these cells.

Organization • Team of three students • Broad division of tasks – Fabrication of device (typically takes 2 hours one day and 2 hours the next day to make 8 devices) – Voltage vs. Current density measurements: this currently takes an hour per sample. We would like to amplify the current and expedite this process. – Calculation of efficiency (1 hour) – Optical Properties (still being developed)

Timeline • Proposals – Who is in your team and what is the team going to do? (April 23) – Include the times when you are available to work on the project outside of class time. Indicate preference for an early start or late start (depends on what you are doing also). • Progress Report 1 – one week after you start work. • Progress Report 2 – two weeks after you start work (this could be your final report)

Timeline • Final write-up due last week of the quarter (TBD) • Short presentation (show and tell) last week of quarter. • Content Post test and CURE post-survey + supplemental survey – last week of quarter.